The recovery flow for A/B devices allows to sideload an OTA downloaded
to a desktop and apply from recovery. This patch allows the "recovery"
context to perform all the operations required to apply an update as
update_engine would do in the background. These rules are now extracted
into a new attributte called update_engine_common shared between
recovery and update_engine.
Bug: 27178350
Change-Id: I97b301cb2c039fb002e8ebfb23c3599463ced03a
